Major morbidity following aortocoronary vein bypass graft surgery (ACBS) is caused by graft occlusion (approximately 25 percent incidence at 1 year) and myocardial infarction. Recent evidence suggests that platelet mechanisms may be responsible for vein graft occlusion both early and late after surgery and that platelet inhibitor drugs may reduce the graft occlusion rate and the vein graft intimal proliferation. Recent data suggest that aortocoronary vein graft occlusion is particularly likely to occur in patients with a shortened platelet survival time. Shortened platelet survival occurs in 50-60 percent of patients with coronary artery disease and may be partially or completely corrected with platelet inhibitor drugs. Patients at high risk of vein graft occlusion may be identifiable prior to coronary surgery by measurement of platelet survival time. The plasma platelet factor 4 level may correlate inversely with platelet survival and would be a faster and more economical means of identifying patients at high risk of vein graft occlusion. A prospective, double-blind, platelet inhibitor, drug trial with Dipyridamole and Aspirin versus placebo (lactose) will be conducted using random allocation to treatment and control groups of patients undergoing ACBS for relief of angina pectoris. The objectives are to evaluate the effectiveness of platelet inhibitor drugs in reducing the incidence of aortocoronary vein graft thrombotic occlusion that occurs in the first two weeks postoperatively and aortocoronary vein graft occlusive intimal proliferation that occurs in the first postoperative year. Vein graft angiography will be performed within the first two weeks after surgery and one year after surgery to determine vein graft patency and mean internal diameter which will be measured from roentgenograms by operator-interactive computer analysis. Platelet survival with 51chromium labeled autologous platelets and plasma levels of platelet factor 4 will be determined in patients in the treatment and control groups before and one year after ACBS.